Fix IllegalAccessError in AnvilInventoryImpl

[bwachtendorf]

On paper minecraft 26.2 I was getting following error when trying to open an anvil GUI:

java.lang.IllegalAccessError: class com.github.stefvanschie.inventoryframework.nms.v26_1.AnvilInventoryImpl$ContainerAnvilImpl tried to access private field net.minecraft.world.inventory.Slot.slot (com.github.stefvanschie.inventoryframework.nms.v26_1.AnvilInventoryImpl$ContainerAnvilImpl is in unnamed module of loader 'WaystoneWarps-1.1.0-SNAPSHOT.jar' @5e9ada49; net.minecraft.world.inventory.Slot is in unnamed module of loader java.net.URLClassLoader @433c675d)
   at WaystoneWarps-1.1.0-SNAPSHOT.jar//com.github.stefvanschie.inventoryframework.nms.v26_1.AnvilInventoryImpl$ContainerAnvilImpl.updateSlot(AnvilInventoryImpl.java:223) ~[?:?]
   at WaystoneWarps-1.1.0-SNAPSHOT.jar//com.github.stefvanschie.inventoryframework.nms.v26_1.AnvilInventoryImpl$ContainerAnvilImpl.<init>(AnvilInventoryImpl.java:134) ~[?:?]
   at WaystoneWarps-1.1.0-SNAPSHOT.jar//com.github.stefvanschie.inventoryframework.nms.v26_1.AnvilInventoryImpl$1$1.createMenu(AnvilInventoryImpl.java:60) ~[?:?]

This fixes the issue by using the appropriate public function.

[stefvanschie]

Regression caused by PaperMC/Paper@0ae1b42.